Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto to the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semi-conductor wafer during the fabrication of semi-conductor devices on the wafer. A wafer is "planarized" or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel.
Polishing heads of the type described above used in the CMP process are shown in U.S. Pat. No. 4,141,180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon et al; and, U.S. Pat. No. 5,643,061 to Jackson, et al.
It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. This problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent, however, consistency and reliability in achieving flatness and uniformity continues to be a problem, in part, because of the lack of control of the polishing head. Heretofore, the pneumatic control systems used in the polishing heads have been relatively unsophisticated, with little or no provision being made for precise control of air pressure applied to the subsystems of the head, and virtually no means provided for testing the head to ensure that all sub-systems are working properly, and within the desired specifications.
Accordingly, there is a clear need in the art for an improved CMP polishing head and method of testing same which overcomes each of the deficiencies discussed above.